This invention concerns a probe for use in an ultrasonic diagnostic apparatus and, more specifically, it relates to an ultrasonic liquid medium for use in a probe adapted to use a liquid medium with a sonic velocity being lower than that in a living body between the oscillator and the living body for increasing the angle of deflection of the ultrasonic beam.
Fluorine-incorporated hydrocarbons, have been proposed as a liquid medium with a sonic velocity therein significantly lower than that in the living body and capable of attaining impedance matching easily with the living body (Japanese Patent Laid-open Publication (KOKAI) No. 85491/1978). However, the material of this kind is easily evaporable having a high vapor pressure of 1-230 torr (25.degree. C.), for example, as seen in FC-72, FC-77, FC-104, FC-75, FC-40 and FC-43. In addition, their high gas-solubilizing effect leads to a defect of tending to form gas bubbles therein when applied to the ultrasonic probe. It is also apparent that commercially available fluorinated hydrocarbons are easily evaporable due to their low boiling points such as 57.7.degree. C. for C.sub.6 F.sub.14, 125.5.degree. C. for C.sub.9 F.sub.20, 47.6.degree. C. for C.sub.2 Cl.sub.3 F.sub.3 and 97.2.degree. C. for C.sub.8 F.sub.16 O at 1 atm.